The proposed project involves the development of a light weight, portable, low cost, variable resistance exercise device (VRED) for use in muscle strengthening in rehabilitation. The aims of the project are to develop a prototype VRED that incorporates magnetorheologic (MR) fluid that changes its viscosity in response to an applied magnetic field, to existing rehabilitation devices, such as knee braces. By incorporating microprocessor technology, the VRED will allow a therapist to program the resistance applied to a patient's muscles in a precisely controlled manner by modifying a very small amount of current from a simple battery. The force and range of motion through which the resistance will be applied are easily controlled through feedback from motion and velocity sensors. Other features of the VRED will include adjustability, measurement of torque and movement velocity with digital display, variable motion stops, and a compliance meter. The long-term objective of the project is to develop a device that will be appropriate for strengthening many muscle groups, however, quadriceps and hamstring muscle strengthening will serve as the model for the proposed study. During the development of the prototype VRED isometric resistance profiles over a 15-90o range will be generated in a group of older men and women and will serve to develop the controllers for the VRED. After the prototype VRED is fabricated the feasibility of the device will be tested on a small group of healthy older individuals, age 50-80 years old. Patient satisfaction will be assessed with regard to appearance; comfort; ease of use; pain during knee flexion and extension; skin irritation; sense of effort. Finally, the VRED will be used in a small pilot strengthening study in people with osteoarthritis of the knee and quadriceps femoris weakness following arthroscopic debridement. Functional outcome measures (Get-up-and-go test and timed-stair climbing) and three dimensional motion analysis during walking will be used to assess the functional outcome of strengthening with the VRED. Data collected during this project will serve as pilot data for further research proposals.